The subject matter disclosed herein relates generally to baffles for impingement cooling of airfoils for gas turbine engines and, more particularly, to a method and casting core for forming a landing for welding a baffle inserted in an airfoil.
Gas turbine engines, such as those used to power modern commercial aircraft, to power sea vessels, to generate electrical power, and in industrial applications, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor. In operation of a gas turbine engine, fuel is injected into and combusted in the combustor in compressed air from the compressor thereby generating high-temperature combustion exhaust gases, which pass through the turbine in order to produce rotational shaft power. The shaft power is used to turn a turbine for driving a compressor to provide air to the combustion process to generate the high energy gases. Additionally, the shaft power is used to power a secondary turbine to, for example, drive a generator for producing electricity, or to produce high momentum gases for producing thrust.
The turbine includes a plurality of turbine stages, wherein each stage includes a stator section formed by a row of stationary vanes followed by a rotor section formed by a row of rotating blades. In each turbine stage, the upstream row of stationary vanes directs the combustion exhaust gases against the downstream row of blades. In order to produce gases having sufficient energy to drive both the compressor and the secondary turbine, it is necessary to compress the air to elevated temperatures and to combust the air, which again increases the temperature. Thus, the vanes and blades, each having an airfoil, are subjected to extremely high temperatures of the combustion exhaust gases, often times exceeding the melting point of the alloys used to make the airfoils.
The airfoils are maintained at temperatures below their melting point by, among other things, cooling the airfoils with a supply of relatively cooler air that is typically siphoned from the compressor. The cooling air is directed into the blade or vane to provide cooling of the airfoil through various modes, including impingement cooling. Specifically, the cooling air is passed into an interior of the airfoil through one or more cooling cavities in the airfoil to remove heat from the alloy. In some implementations, the cooling air is directed into one or more baffles installed within the cooling cavities in the airfoil and having a plurality of cooling holes. Cooling air flowing through the cooling holes of the baffle impinges on and flows against an interior surface of the airfoil. The cooling air then moves through film cooling holes in the airfoil to pass over the outer surface of the airfoil to prevent the hot gases from contacting the vane or blade.
As a general matter, the cooling air effectiveness of the baffle in cooling the airfoil is dependent upon the size of the impingement distance between the side portion of the baffle and the airfoil. Typically, there is an optimum distance between the baffle and the airfoil where cooling effectiveness is maximized. If the distance between the baffle and the airfoil is greater than the optimum distance, the cooling air disperses too much and the cooling effectiveness decreases. On the other hand, when the distance between the baffle and the airfoil is less than the optimum distance, the cooling air does not turbulate or expand sufficiently and the cooling effectiveness decreases. Also, when there are two baffles disposed in the same cooling cavity (i.e., one welded to the outer platform and one welded to the inner platform), the cooling air effectiveness of the baffles is affected by the distance (or gap) between the bottom of the two baffles inside the cooling cavity of the airfoil, with a smaller gap providing a greater amount of impingement cooling than a larger gap.
Baffles are hollow, sheet metal sock structures manufactured separately from the airfoil and later welded to landings on the platforms of the blades or vanes provided for that purpose. The blades or vanes, including the platforms and the airfoil, are typically manufactured using investment casting, a known technique for forming metallic components having complex geometries, especially hollow gas turbine engine components. During the investment casting process, the surfaces of platforms of the blades or vanes, including the baffle landings, are initially formed by wax, which can result in relatively high tolerances for the distance between the platforms that, in turn, determines the length of the airfoil cooling cavity into which the baffles are inserted. In addition, the surface profiles of the baffle landings initially formed on the platforms by wax are undulated, also resulting in relatively high surface tolerances. The relatively high tolerances for these landings can create problems when inserting the baffles into the cooling cavities of the airfoil.
For example, these relatively high tolerances can create relatively significant variability in the gap left between the baffle and the airfoil when the baffle is welded to the platform baffle landings. This is of particular concern where the cooling cavity is conic with large fillets and the baffle follows the conic shape of the cooling cavity. These relatively high tolerances can also create relatively significant variability in the gap left between two baffles when welded to the baffle landings at opposite ends of the cooling cavity. These gaps between the baffles and the airfoil and between the baffles themselves affect the cooling air effectiveness of the baffles as discussed previously. Similarly, if the distance between the inner platform and outer platform is too small as a result of the cumulative tolerances, there may not be sufficient length in the cooling cavity to insert two baffles as they would clash in the cooling cavity rather than leaving a gap. There is, therefore, a need to provide less variability in the location of the baffles inserted into the cooling cavities of airfoils when the baffles are welded to the baffle landings of the blades or vanes.